Enabling Inkjet Printed Graphene for Ion Selective Electrodes with

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Enabling Inkjet Printed Graphene for Ion Selective Electrodes with Post-Print Thermal Annealing Qing He, Suprem R. Das, Nathaniel T Garland, Dapeng Jing, John A. Hondred, Allison A. Cargill, Shaowei Ding, Chandran Karunakaran, and Jonathan C. Claussen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00092 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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ACS Applied Materials & Interfaces

Enabling Inkjet Printed Graphene for Ion Selective Electrodes with Post-Print Thermal Annealing Qing He†,§, Suprem R. Das†,‡,§, Nathaniel T. Garland†, Dapeng Jing#, John A. Hondred†, Allison A. Cargill†, Shaowei Ding†, Chandran Karunakaran⊥, Jonathan C. Claussen†,‡,* †

Mechanical Engineering Department, Iowa State University, Ames, Iowa 50011, USA

‡

Ames Laboratory, Ames, Iowa 50011, USA

#

Materials Analysis and Research Laboratory, Iowa State University, Ames, IA 50010, USA

⊥Biomedical

Research Laboratory, Department of Chemistry, VHNSN College (Autonomous),

Virudhunagar -626 001, Tamil Nadu, India §

These authors contributed equally.

*

Corresponding author email: [email protected]

KEYWORDS: graphene, inkjet printing, thermal annealing, ion selective electrode, potassium, potentiometry

ACS Paragon Plus Environment

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ABSTRACT Inkjet printed graphene (IPG) has recently shown tremendous promise in reducing the cost and complexity of graphene circuit fabrication. Herein we demonstrate, for the first time, the fabrication of an ion selective electrode (ISE) with IPG. A thermal annealing process in a nitrogen ambient environment converts the IPG into a highly conductive electrode (sheet resistance changes from 52.8 ± 7.4 MΩ/☐ for unannealed graphene to 172.7 ± 33.3Ω/☐ graphene annealed at 950°C).

for

Raman spectroscopy and field emission scanning electron

microscopy (FESEM) analysis reveals that the printed graphene flakes begin to smooth at an annealing temperature of 500°C and then become more porous and more electrically conductive when annealed at temperatures of 650°C and above. The resultant thermally annealed, IPG electrodes are converted into potassium ISEs via functionalization with a polyvinyl chloride (PVC) membrane and valinomycin ionophore.

The developed potassium ISE displays a wide

linear sensing range (0.01mM to 100mM), a low detection limit (7 µM), minimal drift (8.6 × 10-6 V/s), and a negligible interference during electrochemical potassium sensing against the backdrop of interfering ions [i.e., sodium (Na), magnesium (Mg), and calcium (Ca)] and artificial eccrine perspiration. Thus, the IPG ISE shows potential for potassium detection in a wide variety of human fluids including plasma, serum, and sweat.


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INTRODUCTION The incorporation of both single-layer and multi-layer graphene into electrodes has significantly improved the sensitivity, detection limit, response time, and biocompatibility of electrochemical sensors and biosensors.1-2 The enhanced performance of graphene-based sensors/biosensors3-7 are attributed to the unique and advantageous material properties of graphene including high electron mobility of up to 200,000 cm2 V−1 s−1, high nominal surface area of 2630 m2 g−1, and high tensile strength of 42 N m−1 as well as the relative ease or conduciveness to functionalize graphene with biorecognition agents. Furthermore, the electrochemical properties of graphene and other graphitic materials in general (e.g., highly ordered pyrolytic graphite (HOPG), graphene oxide (GO), carbon nanotubes (CNTs) and graphite) can be enhanced by inducing edge plane like-sites/defects and defect site functional groups onto the carbon surface through a variety of techniques (e.g., plasma etching, ion bombardment, and wet etching) to increase heterogeneous electron transfer rates and hence improve sensitivity/detection limits of electrochemical sensors.8-12 Additional graphene modification techniques such as nitrogen doping

13-14

and metallic nanoparticle integration15-16

have also been developed to improve graphene sensor performance. Indeed the use of graphene and ‘modified’ graphene have shown tremendous promise for electrochemical sensing, but the high cost and complexity of graphene electrode fabrication (fabrication that often requires lithographic patterning and high temperature chemical vapor deposition in a vacuum chamber reactor) and subsequent chemical modification steps has impeded their implementation and commercialization in a wide variety of in-field and point-of-care applications.17 IPG has recently shown tremendous promise in reducing the cost and complexity of graphene circuit fabrication.18-19 Graphene used in inkjet printing can be synthesized via low


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cost solvent-exfoliation processes to rapidly produce large batches of graphene or graphene oxide flakes that can be subsequently solubilized and formulated into a printable ink.20 Furthermore, the ink jet printing process can be used to make microcircuits with line resolution of approximately 60 µm—thus eliminating the need for UV lithographic techniques that utilize a pre-fabricated photomask with subsequent dry etching of active materials or screen printing techniques that use a pre-fabricated metal stencil to pattern surfaces with defined circuit geometries.21-22 Subsequently inkjet printing has been used for a wide variety of functional devices including thin film transistors,19, 23 acoustic actuators,24 dipole antennas,25 and sensors such as a NO2, Cl2 vapor sensor26 and a temperature sensor27. Herein we develop, for the first time, a solid-contact ISE (potassium selective) with IPG. We further demonstrate how the thermal annealing in a nitrogen ambient can increase/enhance the electrical conductivity, porosity, and nitrogen doping of the IPG electrode parameters to improve the potassium sensing capability of the resultant ISE. EXPERIMENTAL SECTION Reagents Valinomycin (90%), Bis(2-ethylhexyl) sebacate (DOS), potassium tetrakis(4-chlorophenyl) borate (KTClPB), polyvinyl chloride PVC, tetrahydrofuran (THF, 99.8%), sodium chloride (NaCl), sodium sulfate (Na2SO4), sodium bicarbonate (NaHCO3), potassium chloride (KCl), magnesium chloride (MgCl2), sodium phosphate anhydrous monobasic (NaH2PO4), calcium carbonate (CaCO3) and ammonium hydroxide (NH4Cl) were procured from Sigma Aldrich (St. Louis, MO). Spiked sweat containing potential interfering electrolytes, including NaCl, Na2SO4, NaHCO3, KCl, MgCl2, NaH2PO4, CaCO3 and NH4Cl at physiological concentrations

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were

used in the experiments and spiked with potassium concentrations as stated. Artificial eccrine


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perspiration (stabilized at pH 4.5) was purchased from Pickering Laboratory with a listed concentration of ~ 33 mM. The artificial perspiration contains the following metabolites (uric acid, lactic acid, urea, ammonia), minerals (sodium, iron, nitrate, calcium, copper, sulfate, magnesium, potassium, zinc, chloride) and amino acids (glycine, L-histidine, L-serine, Lalanine, L-isoleucine, L-threonine, L-arginine, L-leucine, L-tyrosine, L-asparagine, L-lysine, Lvaline, L-aspartic acid, L-methionine, Taurine, L-citrulline, L-ornithine, L-glutamic, Lphenylalanine) in concentration levels found in real eccrine perspiration. Graphene ink formulation Inkjet printable, graphene-based ink was produced from exfoliated graphene powder, solvents, and the stabilizing polymer ethyl cellulose by modifying previously described methods.20,

22

Briefly, graphene ink batches (20 mL) were synthesized by vortex mixing single layer dispersible graphene (ACS Materials, “completely” reduced graphene oxidase obtained via the Hummer’s Methods) in a mixture of 85% cyclohexanone (Sigma-aldrich 398241) with 15% Terpineol (Sigma-Aldrich T3407) for 1 min at high speed in a 25 mL falcon vortex tube. The initial concentration of graphene to solvent was set to 3.5 mg/ml ratio. Ethyl cellulose (SigmaAldrich 433837) was subsequently added to the mixture at a ratio of 3.5 mg/mL and the subsequent solution was vortex mixed for 5 minutes. The graphene ink was then poured into a 50-mL beaker and probe sonicated (Sonics Vibra-cell VCX-750 ultrasonic processor) at 50% amplitude 3 times for 30 min, bath sonicated for 6 hours at high power, and finally filtered through a 0.45 µm syringe filter to break up and filter out large graphene particles and ensure a consistently smooth, jettable ink with a measured viscosity of 10 cP with a microVISC RheoSense viscometer.


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IPG Electrode Fabrication Graphene ink was inkjet printed via a Dimatrix Materials Printer (Model DMP 2800, Fujifilm) while the electrode patterns were designed in AutoCAD (Auto- desk, San Rafael, CA). To print the ink, 3 mL of the formulated graphene ink was loaded into a Diamatrix printer cartridge with 10 pL nominal drop volume nozzles. Printing was conducted on a 6” silicon wafer placed on the printer plate that was maintained at a temperature of 60°C. The printing speed (8 m/s as verified by the dropwatcher) was set by adjusting the nominal drop spacing (40µm) as well as the nozzle temperature (60°C), waveform, and voltage. The total printed graphene ink layer thickness (i.e., 50 printer passes) on the silicon was measured to be 3.5 µm per a surface profilometer measurement. Thermal annealing and characterization of printed graphene The IPG electrodes were subsequently annealed in a nitrogen environment at varying temperatures (200°C, 350°C, 500°C, 650°C, 800°C and 950°C) within a 2” compact split tube furnace (MTI Corp.) for 1 hour. Subsequent Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and electrical sheet resistance data were acquired for unannealed and thermally annealed samples.

Raman

spectroscopy was acquired with a Renishaw spectrometer microscope using a 488-nm excitation source (argon ion laser), a total acquisition time of 30 s (i.e., three acquisition times of 10 s each), and a 200 µW laser power illumination. The spectrometer was calibrated using an internal silicon reference prior to the measurements. FESEM micrographs were obtained via a FEI Quanta 250 FE-SEM with an electron beam voltage of 10kV. Electrical sheet resistance data were obtained from a signatone four-point probe. The measurements were taken at multiple spots (four different spots) on the sample surface and the average value of these measurements were


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plotted.

The XPS spectra were collected using a Kratos Amicus X-ray Photoelectron

Spectrometer using an Al Kα excitation source (1486.7 eV). The corresponding photoelectron energies from the constituent elements were measured by subtracting the excitation energy from the scan and subsequently obtaining the binding energy survey scan. The N1s spectrum was analyzed for each distinctly annealed IPG electrodes. A Shirley background fitting and Gaussian Lorentzian line peak fitting on the N1s peak spectrum was performed with a CasaXPS software package. Potassium ISE synthesis IPG electrodes were converted into potassium ISEs by depositing a potassium selective membrane onto the graphene. The potassium selective membrane cocktail consisted of 1.0 wt% valinomycin, 66wt% DOS, and 33wt% PVC. The components were dissolved in 1mL THF with concentration of 15wt%. Next, 10µL of cocktail was drop coated onto the IPG electrodes and subsequently dried in air for 6 hours. Potentiometric analysis The potentiometric measurements were performed using a CHI6273E electrochemical workstation (CHI Instruments, USA). An Ag/AgCl electrode (saturated in 3M KCl) was used as the reference electrode. The electrodes were conditioned in 0.01M KCl solution for 24 hours before electrochemical testing and dry stored at room temperature between testing experiments. The analytical performance of the potassium ISEs was analyzed in the concentration range of 108

to 10-2 M via a KCl salt solution. The interfering tests were conducted by following similar

protocols

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where DI water was first spiked with 0.001M KCl, next with 0.01 M KCl, and

finally with artificial eccrine perspiration. IPG ISE drift analysis was performed by constant current chronopotentiometry where the potential was recorded by applying a positive 1 nA


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current for 100 s followed by a negative current of 1 nA for 100 s in a test vial containing 0.1M KCl solution. The drift of the IPG ISE was derived from the potential change during the recording time (∆E/∆t) when the fixed current applied to the IPG ISE.29. RESULTS AND DISCUSSION Potassium IPG ISE fabrication strategy The potassium ISEs were developed on thermally annealed IPG per the process steps displayed in Figure 1. First, graphene ink was formulated with single layer graphene dispersed in solvent (85% cyclohexanone / 15% terpineol) and stabilized with an ethyl cellulose polymer by both bath and probe sonication as noted in the Experimental Section and by modifying existing graphene ink recipes (Figure 1a).20, 22 The formulated graphene ink (viscosity of 10 cP) was inkjet printed via a Dimatrix Materials Printer onto a silicon wafer (with 300 nm silicon oxide) (Figure 1b-d & Experimental Section). Next, a split tube furnace was used to anneal the IPG at 200°C, 350°C, 500°C, 650°C, 800°C and 950°C in a nitrogen environment (i.e., without oxygen) to prevent graphene oxidation during the annealing process (Figure 1e). As shown in subsequent electrical and optical characterization experiments this annealing process is conducted to improve the electrical conductivity of the printed graphene and improve its electrochemical sensing performance.

Finally, a potassium ion-selective cocktail (see

Experimental section for details) is drop coated onto the circular working electrode comprised of IPG to complete the potassium ISE fabrication protocol.


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Figure 1. Schematic fabrication diagram of the IPG potassium ISE showing: (a) graphene ink formulation including bath sonication and probe sonication of the graphene powder mixed with the ethyl cellulose binder and solvents cyclohexanone and terpineol; (b) a syringe (0.45 µm mesh size) is used to filter the ink prior to cartridge loading; (c) loading of cartridge with graphene ink onto the inkjet printer; (d) inkjet printing of the graphene electrodes on a Si/SiO2 (300nm) wafer; (e) thermal annealing of the as-fabricated Si/SiO2/graphene electrodes in a nitrogen environment; and (f) integration of ion selective membranes on to the circular head of the annealed graphene electrode to form a potassium ISE on the printed graphene.

Characterization of IPG electrodes Before potassium ISE immobilization, the IPG was thermally annealed and characterized via electrical measurements, Raman spectroscopy, and FESEM (Figure 2). First, the electrical sheet resistances of the IPG electrodes annealed at six distinct temperatures, viz., 200°C, 350°C, 500°C, 650°C, 800°C and 950°C were compared with the sheet resistance of the unannealed graphene electrode (Figure 2a). Figure 2a displays an unannealed IPG sheet resistance of 52.8 ± 7.4 MΩ/☐ (± 1 stdev.; n=4) that decreases with increasing annealing temperature until a plateau of approximately 147.7 ± 14.9Ω/☐ (± 1 stdev.; n=3) at 800°C is reached. The sheet resistance slightly increases to 172.7 ± 33.3Ω/☐ (± 1 stdev.; n=3) when the annealing temperature is raised


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Thus, there is more than five orders of magnitude increase in the electrical

conductivity as the graphene is annealed to a temperature of 800°C. This significant increase in electrical conductivity (equivalently, decrease in the sheet resistance) is most likely due to the further reduction of the reduced graphene oxide flakes, graphitic crystal formation, and nitrogen doping as verified in subsequent FESEM, Raman spectroscopy, and XPS analysis.

Figure 2. Post-printing thermal annealing in a nitrogen ambient was used to process the graphene IDEs; (a) Electrical sheet resistance vs. annealing temperature of the graphene ISEs. (b) Raman spectra of unannealed and annealed IPG electrodes at distinct temperatures [200°C (red), 350°C (blue), 500°C (pink), 650°C (green), 800°C (dark blue) and 950°C (purple)]. The D, G, as well as 2D peaks are noted in each IPG electrode. FESEM images (lower row, c-i) show the microstructures of the unannealed and thermally annealed printed graphene (color coded boxes around FESEMs correspond to the annealing temperature color legend in Figure 2a and 2b). Raman spectroscopy was performed on both annealed and unannealed IPG samples (Figure 2b). Raman spectroscopy was employed to analyze the printed graphene as it is has been extensively used to characterize both graphene and graphitic materials.30 Single crystal micromechanically exfoliated graphene possesses two characteristic peaks, namely a G peak and 2D peak that correspond to wave numbers of approximately 1580 cm-1 and 2700 cm-1 respectively.


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Furthermore, edge defect, grain boundary, and/or topological defects in the graphene gives rise to the D peak observed at ~ 1350 cm-1 as often portrayed in jagged petal-like growth of CVD grown multilayered graphene.15,

31

In the IPG presented here, the D, G, and 2D peaks are

observed at wave numbers of approximately 1350 cm-1, 1590 cm-1 and 2700 cm-1 respectively. Overall, these three graphene characteristic peaks can be attributed to a defect-rich multilayer graphene stack (the reduced intensity of the 2D peak intensity compared to that of the G peak reveals a multilayer structure, while the D peak intensity reveals a high degree of defects).32 Also, the D and G peaks are red shifted by several tens of wave numbers (the D and G peaks red shifts by 10-20 cm-1) as compared to the typical Raman spectrum for single-layer, single-crystal graphene. This red shift can be attributed to the combination of defects and thermally induced tensile strain in the printed graphene structure as the red shift of the D peak increases with increasing annealing temperature and the red shift of the G and 2D peaks remains stable and temperature independent. Such increasing strain could originate from movement in the defect sites and flake/flake junctions of the printed graphene layers during thermal annealing.

In

addition to one phonon defect-assisted process, there were also multi-phonon defect-assisted processes such as D+D′ peak present in all the electrodes (Figure S1 in the Supplemental Information).32 The microstructure of the IPG electrodes was also characterized via FESEM (Figure 2c) for both unnanealed IPG and IPG annealed at distinct temperatures (200°C, 350°C, 500°C, 650°C, 800°C and 950°C). The FESEM micrographs reveal the relative roughness of the graphene microstructure of the unannealed IPG. This microstructure does not noticeably change during thermal annealing at 200°C and 350°C (Figure 2d & 2e). However, upon reaching an annealing temperature of 500°C, the graphene microstructure is noticeably more smooth with


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less exposed graphene flake edges (Figure 2f). At these higher annealing temperatures (500°C), the individual flakes appear to merge together making a smooth microstructure. This ‘thermally induced smoothening’ of the microstructure of the IPG electrode exhibits more than four orders of conductivity enhancement compared to the unannealed electrode as the physical boundaries between individual graphene flakes becomes “welded” together and the defects are minimized. Further increase in the annealing temperature (650°C or greater, Figure 2g – 2i) displays a more porous microstructure in the electrode (see Figure S2 in the Supplemental Section). At these higher annealing temperatures, the D peak of the IPG Raman spectra increases and the sheet resistance further decreases. As the IPG annealing temperature is further increased (800°C or greater) the IPG achieves both a relatively high electrical conductivity and high number of defects that are necessary for fast heterogeneous charge transport which can subsequently lead to highly sensitive electrochemical sensing/biosensing. The presence of more defects at higher temperatures, relating to the porous micro/nano structure, is further evidenced from the Raman spectra at higher temperature (increasing trend of the ratio of D peak intensity to G peak intensity). To further probe the local electronic structure, the N 1s photoelectron spectra of all the annealed electrodes under consideration were analyzed via XPS (Figure 3). Note that an unannealed sample does not have a nitrogen peak as shown previously.33 However, all the annealed electrodes, including the IPG annealed at 200°C, contain nitrogen (Figure 3). Figure 3d shows the total nitrogen atomic percentage doped in graphene with minimum of 0.4 at. % at 200 °C and maximum of 1.1 at. % at 350 °C. Such N 1s peaks at 400 eV (blue lines/points in Figure 3) are consistent with nitrogen bonded with graphene lattice defects (nitrogen atom substitutional doped in graphene lattice)34-35. It is interesting to note that the formation of nitrogen doping


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within the graphene lattice (alternatively called ‘quaternary nitrogen’) has been realized before in reduced graphene oxide matrices using ammonia annealing at a temperature of 900°C, however in this work the IPG electrodes exhibits nitrogen doped graphene (NG) at much lower temperatures (200°C) in a nitrogen annealed ambient. A higher binding energy N 1s component can also be found at 402 eV (red lines/points in Figure 3) in the thermally annealed IPG which is consistent with nitrogen that contains a higher coordination number. Such higher binding energy N 1s components are presumably formed via substitution with the carbon atoms in the graphene lattice (this type of coordination has been reported to be an ‘oxidized nitrogen’ phase34, 36). The higher binding energy component gains more intensity as annealing temperature increases, however, the relative nitrogen doping in graphene displays a decreasing graphitic coordination with increasing temperature (Figure 3c). Finally, to collectively understand the role of nitrogen annealing at various temperatures from a defect generation standpoint the (ID/IG) intensity ratio from the Raman spectroscopy data originally displayed in Figure 2b was plotted versus total atomic nitrogen concentration (see Figure 3d). The IPG defects, as denoted in the ID/IG ratio plot (greater ID/IG signifies more defects and vice versa), continue to rise with an increasing rate according to annealing temperature while the total nitrogen doping increases to a maximum (350°C) and then decreases with higher annealing temperatures. Thus, the concomitance of increasing superficial defects and a lower level of overall nitrogen composition, i.e., a more graphitic surface, may have led to higher electrical conductivity in high temperature annealed IPG (800°C or higher) as well as to the higher electrochemical potassium sensing capability (see subsequent sections).


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Figure 3. (a) X-ray photoelectron spectroscopy of IPG annealed at various temperatures show emergence of two peaks, a N 1s peak at 400 eV (blue line, peak 1) and a higher order N1 s peak at 402 eV (red line, peak 2); (b) the position of the N 1s peak 1 and peak 2 with respect to annealing temperature; (c) the relative atomic percentage of nitrogen doping on the IPG with respect to annealing temperature and (d) variation of Raman intensity ratio (ID/IG) and total nitrogen doping (atomic percentage) with respect to temperature.

Electrochemical analysis of potassium ISE The IPG was converted into a potassium ion selective electrode by drop coating a potassium selective membrane cocktail (containing valinomycin as the potassium ionophore)


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onto the graphene electrodes that were thermally annealed at 200°C, 350°C, 500°C, 650°C, 800°C and 950°C (see Experimental Section, also note the unannealed IPG electrodes were not sufficiently conductive for electrochemical sensing and hence were not tested in these experiments). The electrochemical ionic response to potassium of the IPG ISE was measured in DI water with various concentrations of KCl solutions vs. an Ag/AgCl reference electrode (Experimental Section). Experimental results show that with increasing annealing temperature, the IPG ISE sensors achieve lower detection limits (Figure 4a) and increased sensitivity (Figure 4b). More specifically, the detection limit of the IPG ISE sensor steadily decreases from 22 µM (Log10K+=10-4.6) to 7 µM (Log10K+=10-5.2) as the annealing temperature increases to 950°C. The sensitivity values of IPG ISEs also increase and display less variability from 48.6 mV to 57.6 mV with increasing annealing temperature. Thus, when the annealing temperature of IPG electrodes reaches 800°C, the ISE sensors begin to exhibit a sensitivity value close to that predicted by the Nernstian equation. Furthermore the overall sensor drift (Figure 4c), as measured via chronopotentiometry37, continued to decrease to 8.6 × 10-6 V/s as the annealing temperature increased to 800°C.

Figure 4. Potassium ion sensing with the IPG ISEs annealed at distinct temperatures (200°C, 350°C, 500°C, 650°C, 800°C, and 950°C). Graphs depicting the average sensor characteristics


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i.e., (a) detection limit, (b) sensitivity (slope of sensing range) and (c) the drift with standard error (n=3).

The decreases in the IPG ISE detection limit and drift with higher annealing temperature could be explained in part by the smoother thermal annealed IPG surface minimizing the spontaneous formation of a water layer between the ISE membrane and IPG electrode. Such a water layer acts as an electrolyte reservoir which re-equilibrates on each sample composition change, consequently introducing potential instability and higher detection limits38. At annealing temperatures of 200°C and 350°C, the rough IPG microstructured surface me more conducive to water layer formation within the more rugged graphene flake topology. This water layer formation could explain the higher variation of sensitivity value for IPG ISEs annealed at the lower temperatures of 200°C and 350°C. However, as the annealing temperature increases, the microstructure of the graphene flakes becomes much more smooth which in turn could reduce the formation of water layer build-up and hence lower the drift and detection limit of the ISE. Based on the electrochemical characterization, the ISE based on IPG thermally annealed at 950°C were chosen for further characterization. This graphene potassium ISE exhibited a Nernstian response to KCl corresponding to a sensitivity value of 57.6 mV per decade of K+ concentration, a characteristic that is predicted by the theory for solvent polymeric membranes doped with valinomycin as the potassium ionophore.39 As shown in Figure 4a, the ISE displayed a linear response to K+ concentration (in logarithmic scale) within the KCl concentration range of 0.01mM to 10 mM. The small standard deviation even between the lowest concentrations of potassium (i.e., R.S.D.: 2.27%, n=3: see Figure 4a) yielded a reliable and repeatable observable detection limit of 7 µM (Log10 K+=-5.2). The potential versus log10K+ plot reveals that the sensing response is nearly instantaneous and reaches the stability within 10s, a response time


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faster than that of a liquid-contact ion-selective electrode, especially at lower concentrations.40 (Figure 5a inset). Such a fast response is important for the real time monitoring of rapid changes in potassium concentration in the sweat that can fluctuate quickly according to perspiration rates.41 Furthermore, the IPG ISE was also tested for repeatability by subjecting the sensor to successive changes in KCl solutions with potassium concentrations alternating from 1 mM, 10 mM, and 100 mM, using four oscillation cycles (see Figure S3 in Supporting Information). This improvement in potassium sensitivity and detection limit is most likely due to reduced water layer formation as previously described, put also do to the increasing porosity of the graphene with higher annealing temperatures (see Figure S2 in the Supporting Information). Such higher porosity or edge defects in graphene render the surface more electroactive than pristine basal plane graphene,15 and subsequently most likely led to improvements in the capacitance and charge storage capability of the electrodes and the near Nernstian slope/sensitivity.57-58


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Figure 5. (a) Potassium calibration plot for the IPG electrodes annealed at a temperature of 950°C portraying potential response changes for successive increases in potassium concentration (10-8 M, 10-7.5 M, 10-7 M, 10-6.5 M, 10-6 M, 10-5.5 M, 10-5 M, 10-4.5 M, 10-4 M, 10-3.5 M, 10-3 M, 10-2.5 M, and 10-2 M) in DI water (b) and successive increases in potassium concentration (10-3 M, 10-2.8 M, 10-2.6 M, 10-2.4 M, 10-2.2 M, 10-2 M, 10-1.8 M, 10-1.6 M, 10-1.4 M, 10-1.2 M ) in sweat. Corresponding potential vs. concentration profiles for potassium concentration sensing in (c) DI water and (d) in artificial perspiration showing sensor reproducibility for three distinct potassium calibration plots performed with the same graphene ISE.

The developed potassium ISE was also tested against the backdrop of common ion interferences that are typically found in sweat28. These initial selectivity experiments were conducted in by spiking DI water with said interferents (at concentration levels typically found in sweat) while the potassium concentration was varied over the sweat potassium physiological range of 1 mM to 63 mM (Figure 5b)41. The potassium ISE sensor showed a Nernstian response with the sensitivity value of 54.5 mV per decade of potassium ion concentration (R.S.D.: 0.91%, n=3). The obtained potassium sensitivity obtained was also similar to the sensitivity of K+ ions previously demonstrated in the KCl solution. The small standard deviation of sensitivity value between experiments conducted in KCl solution (R.S.D.: 2.27%, n=3: see Figure 5c) and KCl solution with multiple additive ions (R.S.D.: 0.91%, n=3:

see Figure 5d) showed both

experiments contained a high-level of repeatability. The selectivity of the ISE is mainly determined by the composition of the membrane and not directly influenced by the type of solid contact used.42 The selectivity coefficients, log ± standard deviation (n=3), of common

interfering ions [i.e., sodium (Na), magnesium (Mg), and calcium (Ca)] were also obtained,

according to previously reported protocols42,39 as follows: log =3.57 ± 0.12, log

=3.90 ± 0.08, log = 3.39 ± 0.21. The obtained selectivity coefficient is comparable to other carbon based solid contact ISE.43-46 Furthermore, the IPG ISE was capable of accurately detecting the amount of potassium found within artificial eccrine perspiration (Figure S3 in


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Supplemental Information) where the measured potentiometric response correlated to a potassium concentration of 33.9 ± 2.4mM—a potassium concentration that is close to the listed value of 33 mM (see Figure S4 in Supplemental Information & the Experimental Section). This result demonstrates that the IPG ISE is capable of selectively detecting potassium within a complex matrix containing a combination of 33 metabolites, minerals, and amino acids (see Experimental Section). Finally, the potassium sensing results of the IPG ISE was compared with similar carbonbased electrodes that have been recently published in the research literature (Table 1). For example, the potassium detection limit of the IPG ISE reached Log10K+=10-5.2, which is lower than similar ISEs comprised with nanostructured with carbon fullerene (Log10K+=10-5.0) and graphene on glassy carbon (Log10K+=10-4.5)44. The recorded drift (8.6× 10 V/s) of the IPG ISE is lower than similar ISEs nanostructured with CNTs (1.7× 10V/s) and graphene on glassy carbon (1.2× 10V/s) 44-45. The linear sensing range (0.01mM to 10 mM) of the developed IPG ISE is also on par with similar solid-state potassium ISE that use nanocarbon-based materials as the transduction element 43-47. Table 1. Performance comparison table of nanocarbon-based, solid-state ISEs Electrode

Detection Limit (M)

Slope Linear Range (mV/decade) (M)

Drift (V/s)

Ref

IPG

10-5.2

57.2

10-5~10-2

*8.6 × 10-6

This work

Fullerene

10-5.0

55

10-5~10-2

-

46

CNT

10-5.5

58.4

10-5.5~10-2.5

*1.7 × 10-5

45

Graphene/GC

10-5

59.2

10-4.5~10-1

*1.2 × 10-5

44

CIM carbon

10-5.6

59.5

10-5.2~10-1

Ψ

4.7 × 10-3

47

Porous carbon

10-5.7

57.8

10-5~10-1

14.9 × 10-3

43, 48

ζ

*This reported drift was obtained via chronopotentiometric means (see Experimental Section). Ψ Long range drift test acquired by monitoring the open circuit potential over a timeframe of 70hrs.


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ζ

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Long range drift test acquired by monitoring the open circuit potential over a timeframe of 20hrs.

IPG: Inkjet Printed Graphene CNT: Carbon Nanotube GC: Glassy Carbon CIM carbon: Colloid-Imprinted Mesoporous Carbon

CONCLUSION In summary, we have developed a scalable inkjet printing process for graphene-based ion selective electrodes or IPG ISEs. Before functionalization with a potassium detecting ionophore, the IPG was thermally annealed in a nitrogen environment to improve both the electrical conductivity and electrochemical sensing capability of the resultant IPG ISE. This annealing process improved the electrical sheet resistance of the IPG by several orders of magnitude from 52.8 ± 7.4 MΩ/☐ for unannealed IPG to 147.7± 14.9 Ω/☐ 172.7 ± 33.3 Ω/☐ for IPG annealed at temperatures of 800°C and 950°C respectively. Furthermore, the thermal annealing process created a highly conductive graphene surface that is well-suited for electrochemical sensing as the “welded porous” surface is sufficiently “electroactive” without the need for graphene chemical modification steps such as metallic nanoparticle integration which is often used to increase the electroactive nature of carbon-based electrochemical electrodes.49-53

The IPG

electrodes were subsequently converted into potassium ISEs by functionalizing the graphene surface with the potassium ionophore, valinomycin, drop coated within a PVC matrix. The resultant IPG ISE, thermally annealed at 950°C, displayed a wide linear sensing range (0.01mM to 10 mM) and low detection limit (7 µM) that faired favorably to other potassium ISEs that used a nanocarbon-based transduction element (e.g., graphene on glassy carbon electrodes, carbon nanotubes and mesoporous carbon; see Table 1). Furthermore, the inkjet printing process developed herein presents a scalable nonmanufacturing route for nanostructured ISEs that


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eliminates the need for both the costly fabrication of graphene through chemical or physical vapor deposition, the need for costly electrode patterning through clean room process such as photolithography, and the need to fabricate metal stencils for each new pattern design such as performed in screen printing. The developed potassium IPG ISE was capable of measuring potassium in a complex artificial eccrine perspiration solution that contained a combination of 33 metabolites, minerals, and amino acids and displays a potassium linear sensing range. The IPG ISE is also capable of sensing physiologically relevant concentrations of potassium including those found in plasma and serum where potassium concentrations can range between 3.1 and 4.6 mM and 0.3–0.4 mM respectively 54. Also, the developed IPG ISE could potentially be used to detect plant available potassium levels in soil (1.1-2.2% of the 10 – 20 g of potassium found in a typical kg of soil is plant available55) where a typical soil slurry dilution (1kg of soil per 2 L of water) would yield a potassium concentration range from 1.4 mM to 5.7 mM. Due to the potential to print graphene on flexible, curvilinear surfaces32 as well as the ability to detect concentrations of potassium found in sweat [~1mM56], the developed potassium ISEs may also be well-suited for wearable epidermal sensors that monitor potassium levels from eccrine sweat glands. Of course, in all of these potassium sensing examples, rigorous testing within field conditions will need to be conducted to prove the viability of the IPG ISE in these various environments where temperature, humidity, and interfering species may vary widely. Such rigorous testing is reserved for future work. In summary, the developed potassium IPG ISEs represent a potential scalable, low-cost manufacturing protocol for monitoring potassium in a variety of biomedical and environmental applications.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Raman spectroscopy and FESEM imaging of IPG electrodes, IPG ISE oscillating potassium sensing, IPG ISE performance table and artificial sweat sensing. AUTHOR INFORMATION Corresponding Author *Jonathan C. Claussen is the author to whom correspondence should be addressed. [email protected] ORCID Jonathan C. Claussen: 0000-0001-7065-1077 Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §These authors contributed equally. Funding acknowledgements The authors gratefully acknowledge that this material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 11901762, by the Roy J. Carver Charitable Trust Foundation under award number 15-4615, as well as by the Iowa State University College of Engineering and Department of Mechanical Engineering.


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Table of Contents Graphic


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Enabling Inkjet Printed Graphene for Ion Selective Electrodes with

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